This invention relates to a servo apparatus for automatic speed control of engines, and in particular servo apparatus which is used to operate a vehicle engine speed throttle.
Most automatic speed control systems for vehicles, whether they use a vacuum or electric motor operated throttles servo, have a positive movement of the servo mechanism to open and to close the throttle.
For example, in the case of a vacuum operated servo the servo is operated by two valves which alternatively admit air or evacuate the zero chamber according to the speed and load variations which are being registered by the vehicle speed transducer. Another system which is used with vacuum servos is an oscillating valve which admits air to, or exhausts air from the servo chamber in accordance with valve control impulses having a variable mark space ratio. Similarly, if an electric motor is used, forward or reverse signals may be applied to the motor which cause the motor to run in the required controlled direction.
One of the difficulties in automatic speed control is that the apparatus is required to fail safe. In this regard the vacuum systems are simple since they can easily be arranged by known methods to fail safe.
On the other hand, while motor driven servos have advantages in that they require no vacuum connection to the vehicle engine, they are more difficult to arrange to fail safe. This is as a result of the motor drive system which drives the throttle actuator arm through a system of reduction gears and a clutch. Should failure occur it is necessary that the clutch be speedily disengaged and because of the mechanical complexity of the gear and clutch system, the changes of a failure of the clutch to release properly or disengage speedily enough, are increased. The use of an electric motor driven in one direction only against a restoring or biasing force, avoids this difficulty, but requires a specialised control system to prevent motor hunting. Feedback of motor position is usually provided by a mechanical connection to a sensor such as a potentiometer.